1. Field of the Invention
The present invention relates generally to amplifiers. More specifically, the present invention relates to systems for efficient amplification of signals using Class D or PWM (pulse width modulation) digital amplifiers.
2. Description of the Related Art
Amplifier designers and manufacturers continue to be pressured to reduce costs, improve efficiency, decrease size & power dissipation, improve output signal quality, reduce electromagnetic and radio frequency emissions, and increase tolerance of noise, distortion, & interference. Although there does not appear to be one complete solution, various signal amplification systems and methods have been proposed to address the various problems.
One technique that has been proposed to increase efficiency over traditional linear amplification is pulse-width modulation (PWM). Despite their inherent power efficiency advantages, there are many difficulties that make it difficult for PWM (or Class D) digital amplifiers to achieve high fidelity performance that can compete effectively with conventional linear (or Class AB) analog amplifiers.
With PWM amplifiers, power supply noise, jitter, circuit noise, and non-linearities in the modulating carrier waveform may be modulated onto the PWM output. Furthermore, to better compete with traditional solutions, it is desirable to reduce the sensitivity of PWM amplifiers to these noise and error sources in order to relax overall system requirements and reduce system costs. Sophisticated techniques have been proposed to attack each of these noise components with limited success. In many instances, the proposed solution increases size, complexity and cost.
Amplifier systems and methods also have the problem in various applications of amplifying signals that have a wide dynamic range. An example of such an application is audio power amplification where there may be a wide dynamic range of audio signal content. Depending upon the design, the amplifier apparatus may be saturated and the amplified signals distorted. In such instances, it is highly desirable for the amplifier to saturate gracefully so that the amplifier comes out of saturation while maintaining stability. When the amplifier is operating in a mode that regularly goes into saturation, the amplifier's overload handling characteristics can dominate in the perceived signal quality, and can make an otherwise acceptable output signal unacceptable. Therefore, it is important for the amplifier to come out of saturation as quickly as possible when the overload condition is no longer present. In PWM amplifiers with sophisticated feedback arrangements, it is very difficult to maintain stability during overload conditions since the closed loop dynamics are disrupted.